The present invention relates to laser apparatus and particularly to a temperature stabilizable laser including an athermal mount in which the laser is assembled and which facilitates the temperature stabilization thereof.
The invention is especially suitable in an optical head containing a laser diode which provides a beam of light of sufficient intensity for laser printing purposes. Other applications of the apparatus provided by the invention may be found wherever operation of a laser, particularly a laser diode, outside thermal failure modes is desired. Such application may include optical communications and elsewhere where electrically operated lasers subject to thermal failure modes are used.
Various athermal mounts have been designed for the purpose of maintaining a laser diode at constant temperature with changes in ambient temperature and operating current. These mounts have included provisions for maintaining focus distances so that optical beams remain of desired area in cross-section, thereby maintaining spot size when such beams are used in laser printers. Typically, the designs involve an assembly including the laser diode and thermo-electric cooler. A temperature sensor in the form of a discreet element, usually a thermistor, is located in the assembly and must respond to the temperature of the assembly in order to follow any changes in the temperature of the laser. The thermal resistance between the thermistor and the laser, as well as the thermal mass or capacitance of the assembly, militates against rapid response to any changes in temperature. The temperature stability of the laser is therefore not maintained as closely to a temperature for safe non-failure mode operation of the laser, as is desired.
Another disadvantage of athermal mount designs which have proposed is their susceptibility to thermal gradients in a direction perpendicular to the optical axis along which the laser beam is projected. These thermal gradients complicate the design of the focus compensation mechanism using dissimilar materials for relatively movable members (inside and outside barrels) of an optical assembly containing a lens or lenses which collimates the beam from the laser diode. A design which utilizes inside and outside barrels of dissimilar material is found in a patent to Thomas E. Yates, U.S. Pat. No. 4,948,221 issued Aug. 14, 1990 and in a patent to Sarraf, U.S. Pat. No. 4,993,801 issued Feb.19, 1991.
Various athermal mount designs and designs of circuits for controlling the temperature of the laser in the mount (by supplying current to a thermo-electric or Peltier cooler) are found in the above-mentioned Yates and Sarraf patents and also in the following U.S. Pat. Nos.: Hori et al., 5,042,042, issued Aug. 20, 1991; Fisher et al, 5,029,335, issued Jul. 2, 1991; Levinson, 5,019,769, issued May 28, 1991; Kluitmans et al., 5,005,178, issued Apr. 2, 1991; Clark et al., 4,924,471, issued May 8, 1990; Bosch, 4,922,480, issued May 1, 1990; Pillsbury et al., 4,910,741, issued Mar. 20, 1990; Odagri, 4,884,279, issued Nov. 28, 1989; Shibanuma, 4,803,689, issued Feb. 7, 1989; Kaneko, 4,661,959, issued Apr. 28, 1987; Baer et al., 4,656,635, issued Apr. 7, 1987; Simmons, 4,061,728, issued Dec. 23, 1986; Sawai, 4,604,753, issued Aug. 5, 1986; Kovats et al., 4,399,541, issued Aug. 16, 1983; and Sato et al., 4,338,577, issued Jul. 6, 1982.
The deficiencies in existing athermal mount designs in providing temperature stabilization are exacerbated by the need to bring wiring to the laser diode and the thermo-electric cooler into the mount. Passages for wiring may cause heat leaks and establish thermal gradients which interfere with the performance of the optical compensating mechanism.
It is the principal object of the present invention to provide improved temperature stabilized laser apparatus in which the laser diode is mounted and arranged to decrease thermal resistance and reduce thermal mass or capacitance so as to provide for rapid stabilization of temperature of the laser due to changes in ambient temperature, laser operating current or other parameters.
It is another object of the present invention to provide an improved athermal mount for a laser diode in which the deficiencies and disadvantages mentioned above are substantially eliminated.
It is a further and more specific object of the present invention to provide an improved temperature stabilizable laser, and particularly an athermal mount containing an electrically operated laser, such as a laser diode, wherein the laser and the temperature sensing elements are integral parts of a thick film hybrid integrated circuit.
It is a more particular object of the invention to provide an improved athermal laser diode mount, including a chip having the diode as an integral part thereof, wherein temperature sensing elements are provided preferably using thick film temperature sensitive materials, distributed on the chip so as to respond rapidly to changes in temperature and particularly to changes which are to be corrected by a thermo-electric cooler with which the chip is in contact.
It is a still further and more particular object of the invention to provide an athermal laser diode mount including a thick film hybrid of which the diode is an integral part which facilitates electrical interconnections between the diode and external circuitry by providing terminals along one edge of the chip to which wires may readily be routed in the mount.
It is a still further and more particular object of the invention to provide an athermal laser diode mount in which the laser diode is part of a thick film hybrid integrated circuit wherein the diode and an optical assembly are constrained in a common reference plane perpendicular to the optical axis along which the laser beam projects thereby minimizing thermal gradients which might cause changes in beam location except along the optical axis thereby simplifying the design of the optical assembly of the mount.